I have made a couple attempts at boring a 5" peice of blackwood using a d-bit. my last attempt was the worse. any idea what i may be doing wrong. i am sure there are many things that could go wrong. I have less than .001" runout on my chuck. i am turning my wood down between centers so i can chuck it up in my chuck. My through hole is large enough to allow the workpiece to slide back into if i turn it down to .75" or less. after i turn it down i am using a center drill to start the hole. then i am using a twist bit to drill deep enough to allow the cutting edge go the d-bit to go it.

I have shared this problem, and been equally frustrated, until I invested in a gun drill and mini air compressor! OK, the gun drill cost nearly £100, and the mini compressor another £60-ish, but the results are instant satisfaction, straight down the middle every time, even up to 18" long!! The action of the compressor blowing the swarf away immediately acts to prevent clog up inside and also keeps the cutting tip area cool. When you've finished a hole you can grab the drill with hand and not get scolded- ( but I imagine anyone in 'Health and Safety' will probably be having fits reading this!!! )

Tailstock not in alignment , which will cause your initial pilot drilling to pull off centre. Not trying to teach anyone how to suck eggs but Lathe tailstocks are adjustable and need adjusting... my own has so much side slop in its guide slots these days that I have to 'feel' for Centre.

If the pilot drill shows some vibratory movement when drilling , that means it is not going in straight and the D bit will tend to follow this off centre passage.

Feeding D bits by hand and FEEL the thing cutting, remove very often for clearing the cut away material.

A blocked tool relief angle will cause the D bit to 'hunt' for a line to take.... Blackwood is a very greasy timber that tends to build up Under the cutting edge and sticks to the tool .. so keep the tool clean and sharp and don't push too hard.

The cutting tip of the D bit should be slightly wider than the shaft, by a couple of thou. so that it cuts a hole slightly larger and allows the shaft some clearance as it passes along.

Keep the D bit cool ,I dip in water and use it as a slight lubricant.

Spindle speed 1000-1500 rpm.

BUT most importantly the pilot should be centred well and there should be no vibration felt when you insert the D bit that would suggest you are not starting on centre.

Try again, gently does it, sharp tool, clear the cuttings every 2mm (for small diameters)... and you should go straight through, hardly ever perfect but within a millimeter of centre unless the bore to length ratio is very high.

That run out over just 5" shows something is seriously out of alignment. Centres might line up but is the tailstock actually square to the axis of the lathe.

In addition to what Geoff has posted check that the lathe is level and its needs to be on a solid base. Even a heavy metal lathe can develop a twisted bed if not properly levelled.

'D' bits - should be ground to be half section, rake should not be too steep and it will need removing every 1/4" or less depending upon diameter of drill to clear the chips. Cool the tip using tepid water NOT cold if it getting too hot - thermal shocks are bad for metal and the timber

Thanks i will definitely need to look into the tail stock adjustment. I am not real sure how to do that, but i bet youtube has some pointers on that. I am worried a bit about my base, i built it out 4x4 lumber, and a table top. with several support struts. but it only weighs about 60 or 70 pounds. In the future i hope to be able to invest in a metal stand for it. I may have exerted too much pressure as well it is the first d-bit i have made and i am still trying to figure out the angles exactly.